1. Field of the Invention
The present invention relates to an optical transmission apparatus for transmitting a wavelength-multiplexed optical signal in an optical fiber network, and more particularly an optical transmission apparatus for broadcasting a distribution data by means of a wavelength-multiplexed optical signal.
2. Description of the Related Art
With the progress of broadband in communication infrastructures, communication and broadcasting are merging in recent years. The communication to date has mainly been based on point-to-point transmission in which stations are connected one-to-one. In contrast, 1-to-N broadcast, in which identical signals (distribution data) are distributed toward a plurality of routes, is required in broadcasting.
At present, a backbone communication network is configured of a large-capacity optical fiber transmission system. FIG. 1 to 3 show conventional configuration examples of an optical transmission apparatus for broadcasting the distribution data over such the large-capacity optical fiber transmission system.
For example, according to the configuration shown in FIG. 1, an optical intensity modulator 2 performs optical intensity modulation of the light supplied from a single-wavelength light source 1, which is constituted of a laser diode (LD) oscillating with a certain wavelength (λ1), with the distribution data, and the modulated light is distributed by a star coupler 3 for splitting the optical power of the light in 1-to-N. However, in the backbone optical communication network in recent years, wavelength division multiplexing (WDM) has been introduced to obtain a large capacity, and signal routes are switched according to the wavelengths. In order to perform broadcasting in such a network, it is necessary to carry identical distribution data on a plurality of wavelengths.
As a means for realizing the above method, as depicted in the configurations shown in FIGS. 2A, 2B, there is a known system (FIG. 2A) in which a plurality of single-wavelength light sources 1 oscillating with different wavelengths are prepared, and the light having each wavelength is multiplexed in an optical wavelength multiplexer 4, and then an optical intensity modulator 2 integrally modulates the wavelength-multiplexed light with identical distribution data, and the integrally modulated wavelength-multiplexed light is split in an optical wavelength demultiplexer 5. Also, as another known system (FIG. 2B), optical intensity modulators 2 are provided on a basis of light having different wavelengths, and the identical distribution data are modulated in each optical intensity modulator 2. However, in a present WDM network, wavelength multiplexing of several tens to several hundreds of wavelengths is performed, and therefore a huge cost may be required if an individual light source is to be prepared for each wavelength.
Meanwhile, to cope with such the anxiety, a multi-wavelength light source capable of outputting a plurality of wavelengths using a single light source is under study. In the official gazette of the Japanese Unexamined Patent Publication Nos. Hei-8-234250, Hei-9-244076, 2003-18126, 2003-69502, and 2001-264830, the inventions with respect to a variety of multi-wavelength light sources are disclosed. Among others, as a light source for WDM, a variety of studies have been conducted on a super continuum (SC) light source capable of generating coherent light through a wide wavelength band. The SC light source is a wideband light source utilizing the phenomenon that the spectrum is extremely spread by passing short pulse light (for example, having a pulse width of the order of picoseconds) of high energy (for example, a few watts at a peak value) through a special fiber (nonlinear medium).
FIG. 3 shows an exemplary configuration of an optical transmission apparatus using an SC light source 10. Also, FIGS. 4A-4C show diagrams illustrating characteristics of the light generated from SC light source 10 on a wavelength axis and a time axis, respectively, according to the configuration shown in FIG. 3. In FIG. 3, SC light source 10 includes a short pulse light source 11 and a nonlinear medium 12, and output light (string of repeated pulses) from short pulse light source 11 is made incident to nonlinear medium 12. As shown in FIG. 4A, the output light from short pulse light source 11 is a string of repeated pulses when viewed from the time axis, while the output light has a narrow spectrum deviating out of the wavelength transmission range of an optical wavelength demultiplexer 5, when viewed from the wavelength axis.
As shown in FIG. 4B, when the light output from short pulse light source 11 is incident to nonlinear medium 12, the spectrum of the output light is widely spread on the wavelength axis, including the wavelength transmission range of optical wavelength demultiplexer 5, due to nonlinear phenomena (self-phase modulation, four wave mixing and stimulated Raman scattering) and wavelength dispersion. Also, on the time axis, the pulse width is compressed, and the power is increased for the amount of compression. The light having a spread spectrum shown in FIG. 4B is called SC light. At this time, since the nonlinear phenomena appear more remarkably as the power incident to nonlinear medium 12 increases, the higher the peak power of the incident pulse light is, the more remarkable the spectral spread becomes. On the contrary, when the power is low, the spectral spread is suppressed.
Next, the SC light output from the nonlinear medium 12 is made incident to optical wavelength demultiplexer 5, and cut out on a wavelength component basis by means of a filter. Thereafter, light of each wavelength is modulated in optical intensity modulator 2, and WDM transmission is performed by carrying signals on each wavelength. As shown in FIG. 4C, the light output from optical intensity modulator 2 is intensity-modulated to the light having a predetermined wavelength transmitted through optical wavelength demultiplexer 5. Let optical power at the time of data ‘0’ be Poff, and optical power at the time of data ‘1’ be Pon, Pon/Poff is called an extinction ratio.
Generally, as short pulse light source 11, a mode-locked laser is employed so as to obtain a multi-longitudinal-modes component having good coherence, and as nonlinear medium 12, an optical fiber in which a wavelength dispersion value and a nonlinear coefficient are managed to efficiently generate the nonlinear phenomena. In the WDM transmission of backbone systems today, wavelength (signal) bandwidths of the C-band zone (1,530 nm-1,565 nm) and the L-band zone (1,565 nm-1,625 nm) are often used. To such the above bandwidths, it has been reported that SC light source 10 produces a satisfactory continuous spectrum over 1,450 nm-1,650 nm.
Additionally, in the official gazette of the Japanese Unexamined Patent Publication No. Hei-7-312575, there is disclosed a configuration for the transmission of a main signal superimposed with a sub-signal by use of an intermediate repeater in an optical communication system.
When configuring an optical transmission apparatus for broadcast using such the above SC light source 10, there is a known configuration as shown in FIG. 5. FIG. 5 shows a diagram illustrating an example of the conventional configuration of the optical transmission apparatus for broadcast using SC light source 10. FIG. 6 shows a diagram illustrating a characteristic of SC light source 10 on both the wavelength axis and the time axis, in the configuration shown in FIG. 5.
In FIG. 5, in order to integrally generate identical distribution data having multi-wavelength components, the SC light output from nonlinear medium 12 is intensity-modulated with the distribution data in optical intensity modulator 2, and thereafter, by demultiplexing the intensity-modulated SC light into each wavelength component in optical wavelength demultiplexer 5, the identical distribution data are output from each wavelength port of optical wavelength demultiplexer 5. FIG. 6A shows a characteristic of the light output from short pulse light source 11, FIG. 6B shows a characteristic of the light output from nonlinear medium 12, and FIG. 6C shows a characteristic of the light output from optical intensity modulator 2. Since the identical distribution data are carried on the light having each wavelength, before demultiplexing to each wavelength in optical wavelength demultiplexer 5, it is possible to integrally modulate with the identical distribution data for the entire wavelength bandwidths having been multiplexed.
However, in the conventional configuration shown in FIG. 5, there have been problems described below:
(1) Because the SC light is generated with spreading the spectrum by the pulse compression, the peak power of the pulse increases. For example, the peak power reaches as high as approximately +32 dBm. Considering the maximum tolerance of input light power of optical intensity modulator 2 generally in use, it is difficult to input such the high power pulse without modification. In a LN (LiNO2) optical intensity modulator usually in use, the input upper limit is as high as approximately +20 dBm or of that order. Therefore, it is necessary to decrease the input value in advance using an attenuator so as to avoid break of optical intensity modulator 2. As a result, a wideband optical amplifier is additionally required for the purpose of level compensation.
(2) Because the SC light has a wide spectral width, wavelength dependency of a variety of characteristics of optical intensity modulator 2 (such as a transmission loss and an extinction ratio) becomes a great problem. For example, when the transmission loss characteristic is not uniform (flat) throughout the spectral range of the SC light, the optical power on each wavelength port after being split by optical wavelength demultiplexer 5 is also not uniform. As a result, level adjustment on a wavelength (channel) basis becomes separately necessary.
(3). When the SC light is integrally modulated, it is also necessary to pay attention to an influence of wavelength dispersion. FIGS. 7A-7C show diagrams illustrating the wavelength dispersion. The SC light is generated by making output light (pulse width: Δt, and pulse repetition period: T), supplied from short pulse light source 11 shown in FIG. 7A, be incident to nonlinear medium 12. At this time, when the wavelength dispersion of nonlinear medium 12, an optical fiber, is completely zero (refer to FIG. 7B), a group delay difference in a pulse is not generated even the spectrum becomes spread by means of the SC light. Therefore, each wavelength component can be intensity-modulated in optical intensity modulator 2 at the identical temporal timing.
On the other hand, when the wavelength dispersion is existent (refer to FIG. 7C), the group delay difference occurs in the wavelength band of the SC light. As a result, time deviation arises between the wavelength components before the SC light is input into optical intensity modulator 2. By performing integral intensity modulation, there is produced a wavelength range in which the modulation cannot be performed correctly in optical intensity modulator 2. When the pulse bitrate becomes particularly higher (or the repetition time T becomes shorter), it is necessary to pay attention to the above problem. For example, when a single-mode dispersion-shifted fiber (DSF) having a length of 3 km is used as nonlinear medium 12, considering the dispersion coefficient of a general DSF fiber, a delay difference of approximately 180 ps/km is produced between a zero-dispersion wavelength 1,550 nm and the longest wavelength of 1,625 nm in the L-band, and a delay time of approximately 540 ps arises in case of 3 km. Assuming a case of a bitrate of 10 Gbps, since the repetition time T=100 ps, the delay longer than the repetition time occurs, and integrated modulation becomes impossible.
(4) In the configuration shown in FIG. 5, the pulse extinction ratio (Pon/Poff) is determined by the characteristic of optical intensity modulator 2. Generally, optical intensity modulator 2 cannot completely intercept the light from passing through. Since the SC light has a high peak level of the pulse power, even when the optical power is Poff (in case of data ‘0’), high optical power may pass through as residual light, if the extinction ratio is small.